[llvm] [CMake] Only export the LLVM_LINK_LLVM_DYLIB setting if not yet set (PR #135570)

[Jonas Rembser via llvm-commits]

guitargeek wrote:

Yes that's true, but if you want to make that configurable, the downstream package (in this case e. g. standalone flag would need another flag so you can steer that behavior when building the downstream package. Of course that's also an option, but I think making this change in LLVM is more sustainable because Clang is not the only downstream package affected by this

https://github.com/llvm/llvm-project/pull/135570